"Charter of Diversity"

HZDR is a member of the "Charter of Diversity", an initiative encouraging diversity in business companies and public institutions. It is supported by the German federal government, the chancellor of Germany being its patron.

View of the two THz sources at the ELBE accelerator: The pulses from the diffraction radiator source (right) and from the undulator source (orange part) are transported into the laboratory on the roof by two optical beamlines

Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) succeeded in constructing a prototype facility for research with high terahertz (THz) fields. The performance of the TELBE facility was successfully tested by conducting a time-resolved pilot experiment. The achieved THz fields exceed those of existing THz sources at similar repetition rates by orders of magnitude, although TELBE has not yet reached its design parameters. The results have been published in the journal “Scientific Reports”.

The resonances of many important fundamental excitations are in the THz frequency range. Intense, specifically shaped THz pulses can be utilized to manipulate material properties on ultra-fast timescales selectively as several groundbreaking experiments in the past years could show. Scientists hope to understand complex processes such as high-temperature superconductivity better when analyzing these short-lived exotic THz driven states of matter. Existing THz sources have so far failed to provide optimal parameters for many of the envisioned investigations, e.g. by providing only moderate repetition rates.

The collaboration involving experts from several accelerator laboratories like Deutsches Elektronen-Synchrotron (DESY), Karslruhe Institute of Technology (KIT), SLAC National Accelerator Laboratory and the European XFEL GmbH, now demonstrated that intense THz pulses can be generated at unprecedented repetition rates utilizing a very compact, quasi-cw linear electron accelerator. The concept combines superconducting radio frequency accelerator technology with the superradiant THz emission principle. Two of the results are of particular importance. Firstly, it could be shown that multiple THz sources can be operated in parallel by one accelerator. Thereby future user facilities, like TELBE itself, can provide multiple user groups with THz pulses of individually adjustable parameters. Secondly, during the pilot experiments a timing accuracy between the THz pulses and external laser systems in the 10 femtosecond regime was demonstrated routinely. These measurements originate from an interdisciplinary collaboration between accelerator physicists, laser physicists, material scientists and life scientists which aimed to make the prototype facility at the center for high power radiation sources in Dresden suitable for experiments in diverse research areas right from the start.

Pilot experiment: Coherent spin waves, excited by THz pulses

The chosen pilot experiment investigated the coherent, selective THz excitation of a spin wave in nickel oxide. The high transient magnetic field of the multicycle THz pulses from the TELBE facility couples in this case directly to the electron spins and induces a coherent spin motion. A synchronized femtosecond laser pulse is utilized to measure the spin deflection as a function of time via the Faraday effect. Due to the orders of magnitude higher repetition rate, experiments like this can be performed with much higher accuracy or speed. „It took us all by surprise that we could take such nice data at such an early stage during the commissioning. Once the target parameters of the TELBE facility have been reached we can work with by a factor of 100 stronger THz pulses, which in the case of the spin deflection would yield values close to what would be necessary for inducing an actual spin flip. This would have an enormous technological importance,” says Michael Gensch, corresponding author of the publication.

More pulses per seconds – opportunity for unique experiments

“The high repetition rate enables to employ a number of techniques in combination with the THz pump pulses of which scientists so far could only be dreaming of. We are discussing with our pilot users which additional probe techniques should be established at the TELBE facility. One of the currently investigated options is to implement time resolved UV photoelectron spectroscopy,” says Dr. Michael Gensch.

„Friendly user“ operation starting in summer 2016

The facility is beginning to start “friendly user” operation for a limited number of proposals in summer 2016. “Particularly in the initial phase of such a new facility we cannot always guarantee to provide optimal parameters for individual experiments over typical beamtime durations,” says Dr. Sergey Kovalev, the future beamline scientist at TELBE. “For that purpose we will provide special support during the beamtimes and continue to work on the optimization of the facility.” Deadline for proposals for the first run of “friendly user” operation is April 15, 2016.